1. Field of The Invention
The present invention relates generally to methods and apparatus for the optical inspection of materials. The present invention relates more specifically to a method and apparatus for using laser induced fluorescence to quantitatively measure thin deposits on test surfaces used in thermal oxidation testing for fuels.
2. Background Information
Jet aircraft engines commonly use the fuel flowing in the engine as a coolant in the various heat exchanges used in the engine systems. The unusual high temperatures of the heat exchangers often result in the thermal breakdown of the fuel and the depositing of carbonaceous deposits on the heated surfaces. These deposits degrade the performance of the heat exchangers and can clog important fuel flow lines ultimately resulting in engine breakdown. Jet engine fuels vary in this tendency to create deposits, some having a higher tendency than others. To assure quality control in the industry, thermal oxidation stability tests have been developed for quantifying this characteristic of aircraft fuels.
The present standard thermal oxidation stability tests for jet fuels are defined by the American Society for Testing Material (ASTM) in method D-3241 which utilizes an apparatus based upon U.S. Pat. No. 3,670,561 (Hundere I patent) which disclosure is incorporated herein by reference. The apparatus defined by the Hundere I patent is improved upon by the apparatus disclosed in U.S. Pat. No. 5,101,658 (Hundere II patent) which disclosure is also incorporated herein by reference In the Hundere patents process, a thermal oxidation test heater tube (TOT heater tube), typically an aluminum cylinder, is heated to a specific temperature and fuel under test is flowed over it for a specified time. Carbonaceous deposits from on the heated tube during the test and the maximum thickness of such deposits is used as a measure of the tendency of the fuel to break down and leave these deposits.
The level of tube deposits are presently rated by either the Mark 8A Tube Deposit Rater (available from Alcor, Inc., 10130 Jones Maltsberger Road, P.O. Box 32516, San Antonio, Tex. 78284) or a Tuberater (available from Erdco Engineering Corp., 721 Custer Avenue, Evanston, Ill. 60202), and used in conjunction with the ASTM color standard No. 12-416600-00. In the visual technique of rating the test results, the procedure for measuring the deposits involves an individual visually comparing the deposits on the metal tube with the ASTM color standard. This visual comparison method has several problems. It is not uncommon for different test operators to select different color values and, therefore, arrive at different fuel ratings. The color standards differ in color from the typical tube deposit color and the tubes themselves are round while the color standard is on a flat substrate. All of these subjective factors, therefore, tend to confound the fuel rating It would be desirable to have a more objective means for quantifying the level of tube deposits and thereby for rating the fuels tested U.S. Pat. No. 3,705,014, issued to Townsley, discloses an objective optical method based upon the absorption of light by the tube deposits. The limitation of this invention lies in the difficulty of assessing the absorption of the lightly deposited tubes, grades 1 and 2.
U.S. Pat. No. 3,705,014, issued to Townsley, also discusses the use of light to quantify the deposits of interest on a thermal oxidation tester tube. The principle of operation of the Townsley apparatus, however, depends upon measuring the diminution of the light impinging on the test tube. Since the deposits are a very thin coating on the tube, having only very slight absorption, the method depends upon detecting a small change superimposed on a large optical signal. No mention is made in Townsley of applying fluorescence to this approach to overcome this problem of signal detection.
U.S. Pat. No. 4,866,283, issued to Hill, discloses an optical inspection system using laser induced luminescence to detect the quality of organic materials on food products. The Hill patent, method, and apparatus, however, incorporates and requires the use of complex spectrometer optics and circuitry to analyze the frequency and wave length of the induced fluorescence. Hill does not address the quantification of thin deposits of trace materials on metal surfaces.
U.S. Pat. No. 4,802,762, also issued to Hill, describes an optical inspection system for using laser induced luminescence to detect the deterioration of a polymer base material. Here again, however, the apparatus and method require the use of a spectrometer for accurate indications of the physical characteristics of the specimen. Hill does not address the art of quantifying thin deposits of carbonaceous materials such as are found in thermal oxidation tests. The Hill apparatus is intended to quantify changes in bulk samples.